Sedimentary phosphate deposits: ore formation and exploration

Sedimentary phosphorite is a P-rich, bioelemental sediment commonly associated with coastal upwelling and an important fertilizer ore. Because the deposition of phosphorite is so closely linked to biology, periods of enhanced... [ view full abstract ]

Sedimentary phosphorite is a P-rich, bioelemental sediment commonly associated with coastal upwelling and an important fertilizer ore. Because the deposition of phosphorite is so closely linked to biology, periods of enhanced accumulation generally correspond to prominent changes in the biogeochemical cycling of P through time. Such changes have resulted in four globally recognized phosphogenic episodes. Each represents periods of profound environmental change that modified the feedbacks regulating Earth’s redox state, climate, and biologic evolution.

The first phosphogenic episode is Paleoproterozoic in age (2.2-1.8 Ga) and produced small, sub-economic, peritidal phosphorite occurrences. Neoproterozoic-Paleozoic phosphorites of the second event (ca. 740-410 Ma) are upwelling-related epeiric sea deposits and the first true phosphorite giants. The third phosphogenic episode began in the Carboniferous and continued through the Permian (305-250 Ma) to produce continental margin and epeiric sea phosphorites. The fourth started in the Jurassic and ended in the Miocene (200-10 Ma) to also create high-grade, upwelling-related phosphorites on continental margins and in epeiric seas.

The fourth episode contains the largest single accumulation of economic phosphorite on Earth. The Cretaceous-Eocene South Tethyan Phosphogenic Province (STPP), which extends from Morocco to the Middle East, possesses at least half of the global phosphate reserve base. It records coincident tectonic, climatic, oceanographic, and sedimentologic extremes that increased the delivery and sequestration of P along the South Tethyan margin. Lagoonal circulation under a semi-arid climate is interpreted to have transported phosphate away from the upwelling front to precipitate carbonate fluorapatite across the entire platform. Resultant pristine phosphorite was hydraulically reworked by storms to form high-grade ore zones of tabular granular phosphorite beds. Amalgamation of these phosphatic tempestites produced the thickest stratiform deposits.

What the STPP reveals is that ore grade is controlled primarily by the interplay between fairweather and storm depositional processes. Pristine phosphorite is a fairweather facies that forms in environments where stratigraphic condensation stabilizes the zone of phosphogenesis in the sediment. The grade and thickness of granular phosphorite is governed by storm strength and frequency. Storm strength dictates the quantity of pristine phosphorite reworked. Storm frequency regulates the amount of pristine phosphorite preserved between phosphatic grainstones, which in turn governs whether thick, amalgamated economic phosphorite develops.

The interaction of these processes and production of phosphatic ore are linked directly to the transgressive systems tract and development of the maximum flooding surface. Thus, the maximum flooding surface is the most important target when designing exploration and production strategies.